Method for transmitting and receiving downlink control information

ABSTRACT

A method for efficiently transmitting and receiving downlink control information is disclosed. The method includes, at a base station, receiving feedback information including a precoding matrix index (PMI) from a user equipment (UE) and transmitting precoding information having a predetermined bit number according to the number of antenna ports and a transmission mode of the base station. The precoding information of a predetermined transmission mode in the precoding information includes confirmation information indicating that the base station uses a PMI which is recently received from the UE.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/646,460, filed Oct. 5, 2012, which is a Continuation of applicationSer. No. 12/382,999, filed Mar. 27, 2009 (now U.S. Pat. No. 8,315,324),and claims priority to U.S. Provisional Application Nos. 61/045,971,filed Apr. 18, 2008; 61/075,303, filed Jun. 24, 2008; 61/077,860, filedJul. 2, 2008; and Korean Patent Application No. 10-2008-0100020, filedOct. 13, 2008, the contents of each of the above-identified applicationsis incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile communication technology, andmore particularly, to a method of efficiently transmitting and receivingdownlink control information (DCI).

2. Discussion of the Related Art

In a next-generation mobile communication system, a high datatransmission rate is required. Thus, various researches into amulti-input multi-output (MIMO) antenna technology are ongoing. First, ageneral MIMO technology will be briefly described.

The MIMO is abbreviated from the term “multi-input multi-output” andindicates a method of employing multiple transmission antennas andmultiple reception antennas so as to improve transmission/reception dataefficiency, instead of a conventional method using one transmissionantenna and one reception antenna. That is, a transmitter or a receiverof a radio communication system uses multiple antennas so as to increasecommunication capacity or improve transmission/reception performance.Hereinafter, the “MIMO” is also referred to as “multi-input multi-outputantenna”.

The multi-input multi-output antenna technology indicates a technologyof collecting data pieces received via several antennas withoutdepending on a single antenna path in order to receive one message.According to the MIMO technology, a data transmission rate is improvedin a specific range or a system range can be increased with respect to aspecific data transmission rate.

Since next-generation mobile communication requires a data transmissionrate which is significantly higher than that of the existing mobilecommunication, an efficient multi-input multi-output antenna technologyis expected to be necessarily required. Under these circumferences, theMIMO communication technology is the next-generation mobilecommunication technology which is widely applicable to mobilecommunication terminals and repeaters. The MIMO technology is attractingattention as the next-generation technology to overcome the restrictedtransmission amount of the mobile communication that has reached thelimit due to the data communication extension.

Among various technologies of improving transmission efficiency whichare currently being researched, the MIMO technology of using multipleantennas in both a transmitter and a receiver is attracting mostattention as a method of remarkably improving communication capacity andtransmission/reception performance with increasing additional frequencyallocation or power consumption.

FIG. 1 is a view showing the configuration of a general MIMO antennacommunication system.

As shown in FIG. 1, if the number of transmission antennas is increasedto N_(T) and, at the same time, the number of reception antennas isincreased to N_(R), a theoretical channel transmission capacity isincreased in proportion to the number of antenna ports, unlike the casewhere multiple antennas are used in only any one of the transmitter andthe receiver. Thus, frequency efficiency can be remarkably improved. Thetransmission rate due to the increase in channel transmission capacitycan be theoretically increased by a value obtained by multiplying amaximum transmission rate R_(o) at the time of using one antenna by arate increasing ratio (R_(i)).

R _(i)=min(N _(T) ,N _(R))  Equation 1

That is, for example, in an MIMO communication technology using fourtransmission antennas and four reception antennas, the transmission rateis theoretically four times that of a single-input single-output antennasystem.

After the theoretical increase in the capacity of the MIMO antennasystem was proved in the mid-1990s, various technologies ofsubstantially improving a data transmission rate have been activelydeveloped up to now. Among them, several technologies have been alreadyapplied to the various radio communication standards such as thethird-generation mobile communication and the next-generation wirelesslocal area network (LAN).

According to the trend of the research into the MIMO antenna up to now,various researches such as researches into information theory related tothe computation of the communication capacity of a MIMO antenna invarious channel environments and multiple access environments,researches into the model and the measurement of the radio channels ofthe MIMO system, and researches into space-time signal processingtechnologies of improving transmission reliability and transmission ratehave been actively conducted.

The MIMO technology includes a spatial diversity method for increasingtransmission reliability using symbols passing through various channelpaths and a spatial multiplexing method for improving a transmissionrate by simultaneously transmitting a plurality of data symbols using aplurality of transmission antennas. Recently, researches into a methodof obtaining the respective advantages of the two methods by combiningthe two above-described methods are ongoing.

Hereinafter, the methods will be described in detail.

First, the spatial diversity method includes a space-time block codingmethod and a space-time trellis coding method using both a diversitygain and a coding gain. Generally, the trellis coding method isexcellent in view of the improvement of a bit error rate and the degreeof freedom for code generation, but the space-time block coding methodis advantageous in that computation complexity is simple. A spatialdiversity gain can be obtained from a product N_(T)×N_(R) of the numberN_(T) of transmission antennas and the number N_(R) of receptionantennas.

Second, the spatial multiplexing method indicates a method oftransmitting different data streams via transmission antennas. At thistime, in a receiver, mutual interference is generated between data whichare transmitted from a transmitter. The receiver eliminates theinterference using an adequate signal processing method and receives thedata. The receiver for eliminating noise, which is used herein, includesa maximum likelihood receiver, a zero forcing (ZF) receiver, a minimummean-squared errors (MMSE) receiver, a Diagonal Bell LaboratoriesLayered Space-Time (D-BLAST) receiver and a Vertical Bell LaboratoryLayered Space-Time (V-BLAST) receiver. In particular, if the transmittercan know channel information, a singular value decomposition (SVD)method may be used.

Third, a combination of the spatial diversity method and the spatialmultiplexing method may be used. If only the spatial diversity gain isobtained, a performance improvement gain according to the increase indiversity order is gradually saturated. If only the spatial multiplexinggain is obtained, the transmission reliability of the radio channeldeteriorates. Accordingly, researches into the methods of obtaining boththe two gains while solving the above-described problems have beenconducted. Among them, a Double Space-Time Transmit Diversity(Double-STTD) or Space-Time Bit Interleaved Coded Modulation (STBICM)may be used.

In the MIMO antenna system, the transmitter performs precoding withrespect to transmission data and transmits the pre-coded data, and thereceiver receives the signal using a precoding vector or a precodingmatrix used in the transmitter.

The precoding matrix for performing the precoding uses a specificprecoding matrix among precoding matrixes which are predefined in theform of a codebook in both the transmitter and the receiver. That is,the receiver feeds back channel information according to the specificprecoding matrix in the predefined codebook to the transmitter, and thetransmitter transmits the signal using the feedback signal.

In downlink transmission, the receiver may be a user equipment (UE) or aterminal and the transmitter may be a base station, a node-B or aneNode-B (hereinafter collectively called as “base station”). Forexample, the UE may report a specific precoding matrix index(hereinafter, referred to as a “PMI”) in the predefined codebook via anuplink channel, and the base station may transmit a downlink signalusing a precoding matrix corresponding to the reported PMI.

There may be a plurality of PMIs which are temporally reported by theUE. Accordingly, confusion about which of the PMIs is used by the basestation may occur. In order to solve this problem, the base stationpreferably transmits control information indicating which of the PMIsreported by the UE is used. If the control information explicitlyindicating which of the PMIs reported by the UE is used by the basestation is transmitted via a downlink channel, the overhead of thedownlink control information may be increased.

Accordingly, there is a need for a technology of efficiently preventingconfusion about a PMI used between the base station and the UE fromoccurring using a small amount of control information.

SUMMARY OF THE INVENTION Technical Problem

An object of the present invention devised to solve the problem lies ona method of efficiently informing a user equipment (UE) of informationabout a precoding matrix index (PMI) used by an base station using asmall amount of control information for precoding, that is, precodinginformation.

In the embodiment, a confirmation information is explicitly defined inprecoding information having a small bit number according to a temporalrelationship between the transmission of the feedback information by theUE and the transmission of the control information by the base station.Another object of the present invention devised to solve the problemlies on a CRC attaching method capable of minimizing the overhead whilesystem performance is maintained, when the above-described method isperformed.

Technical Solution

The object of the present invention can be achieved by providing amethod for transmitting downlink control information by an base station,the method including: receiving feedback information including aprecoding matrix index (PMI) from an user equipment (UE); applying aprecoding to a downlink signal and transmitting the downlink signal; andtransmitting precoding information having a predetermined number of bitsaccording to the number of antenna ports and a transmission mode of thebase station, wherein the precoding information of a specifictransmission mode in the precoding information includes confirmationinformation indicating that the base station applied a precodingaccording to a PMI received from the UE, wherein the precoding to thedownlink signal for a specific resource block (RB) in n^(th) subframe isaccording to a latest PMI received from the UE on or before n-x^(th)subframe, and wherein “x” is a predetermined integer or an integerdetermined by upper layer signaling higher than a physical layer.

In another aspect of the present invention, provided herein is a methodfor receiving downlink control information from an base station by auser equipment (UE), the method including: transmitting feedbackinformation including a precoding matrix index (PMI) to the basestation; receiving a downlink signal from the base station, wherein thebase station has applied a precoding to the downlink signal; andreceiving precoding information having a predetermined number of bitsaccording to the number of antenna ports and a transmission mode of thebase station, wherein the precoding information of a specifictransmission mode includes confirmation information indicating that thebase station applied a precoding according to a PMI transmitted from theUE, wherein the precoding applied by the base station to the downlinksignal for a specific resource block (RB) in n^(th) subframe isaccording to a latest PMI transmitted from the UE on or before n-x^(th)subframe, and wherein “x” is a predetermined integer or an integerdetermined by upper layer signaling higher than a physical layer.

In on embodiment, “x” may be predetermined as 4.

The transmitting/receiving of the feedback information may includetransmitting/receiving the PMI via at least one of a physical uplinkcontrol channel (PUCCH) and a physical uplink shared channel (PUSCH)from the UE, and the confirmation information considers only the PMItransmitted/received via the PUSCH. In addition, a cyclic redundancycheck (CRC) may be attached to the PMI transmitted/received via thePUSCH, and the CRC may not be attached to the PMI transmitted/receivedvia the PUSCH.

The transmitting/receiving of the feedback information may includetransmitting/receiving the PUCCH including the PMI which is piggy-backedto the PUSCH, and the CRC may not be attached to the PMI in the PUCCHwhich is piggy-backed to the PUSCH.

When the feedback information is transmitted/received via the PUSCH, thefeedback information may be transmitted in any one of a single PMItransmission mode or a multi PMI transmission mode, and, only when thefeedback information is transmitted in the multi PMI transmission mode,the CRC may be attached to the PMI.

The precoding information of the specific transmission mode may beprecoding information of a closed-loop spatial multiplexing transmissionmode.

Advantageous Effects

As described above, according to the embodiments of the presentinvention, it is possible to efficiently inform a user equipment (UE) ofinformation about PMI used by an base station using a small amount ofprecoding information. In particular, it is possible to preventconfusion between the base station and the UE from occurring using asmall amount of control information by explicitly defining theconfirmation information in the precoding information having a small bitnumber according to a temporal relationship between the transmission ofthe feedback information of the UE and the transmission of the controlinformation of the base station.

In addition, it is possible to minimize the overhead while maintainingsystem performance by efficiently setting the CRC attachment.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a view showing the configuration of a general multi-inputmulti-output (MIMO) communication system.

FIG. 2 is a flowchart illustrating a processing structure of downlinkcontrol information (DCI).

FIG. 3 is a view showing a temporal relationship between a precodingmatrix index (PMI) reported by a user equipment (UE) and confirmationinformation according to an embodiment of the present invention.

FIG. 4 is a view showing a resource allocation relationship in the casewhere a physical uplink shared channel (PUSCH) in which data, a controlchannel, ACK/NACK and rank information are multiplexed is transmitted.

FIG. 5 is a diagram explaining an example of a method of transmittingdownlink control information by the base station in accordance with oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description set forth below in connection withthe appended drawings is intended as a description of exemplaryembodiments and is not intended to represent the only embodiments inwhich the concepts explained in these embodiments can be practiced.

The detailed description includes details for the purpose of providingan understanding of the present invention. However, it will be apparentto those skilled in the art that these teachings may be implemented andpracticed without these specific details. In some instances, well-knownstructures and devices are omitted in order to avoid obscuring theconcepts of the present invention and the important functions of thestructures and devices are shown in block diagram form. The samereference numbers will be used throughout the drawings to refer to thesame or like parts.

As described above, if information indicating which of precoding matrixindexes (PMIs) reported by a user equipment (UE) is used by an basestation is not informed, confusion about the used PMI between the basestation and the UE occurs and thus system performance may deteriorate.If information about the PMI used by the base station is explicitlydescribed in downlink control information (DCI), the above confusionabout the used PMI does not occur. But serious overhead may occur in arestricted downlink control channel as described above. Accordingly, thepresent invention provides a method of efficiently informing the UE ofthe information about the PMI used by the base station using a smallamount of control information for precoding. Hereinafter, a method oftransmitting DCI in a 3^(rd) Generation Partnership Project Long TermEvolution (3GPP LTE) system will be described. The 3GPP LTE system isonly exemplary and the present invention is applicable to any mobilecommunication system by the same method.

In 3GPP TS 36.212: “Multiplexing and Channel Coding” (release 8), v8.2.0(2008-3), the DCI includes downlink or uplink scheduling information oruplink power control information of one media access control (MAC) ID.The MAC ID may be implicitly encoded by a cyclic redundancy check (CRC).The DCI may be transmitted as follows.

FIG. 2 is a flowchart illustrating a processing structure of DCI.

First, DCI information elements are multiplexed. At this time,multiplexing may be performed according to a method defined in thefollowing DCI format. In 3GPP TS 36.212: “Multiplexing and ChannelCoding” (Release 8), v8.2.0 (2008-3), formats 0, 1, 1A, 1C and 2 aredefined as the DCI format. In more detail, the format 0 may be used forscheduling of physical uplink shared channel (PUSCH), the format 1 maybe used for scheduling of a physical downlink shared channel (PDSCH)codeword, the format 1A may be used for dense scheduling of one PDSCHcodeword, the format 1C may be used for dense allocation ofDownlink-Shared Channel (DL-SCH) allocation, and the format 2 may beused for scheduling of the PDSCH to a UE in a spatial multiplexing mode.

The multiplexed information elements are attached with a CRC forchecking the reception error. The CRC may be attached to only a specificinformation element according to the transmitted information or may beattached by different methods according to the DCI formats. The CRCattached DCI is channel-coded. Thereafter, rate matching is performedwith respect to the channel-coded DCI according to transmission rates.

Among the above-described DCT formats, the format 2 used for schedulingof the PDSCH to the UE in the spatial multiplexing mode will bedescribed in detail.

The DCI format 2 may include a resource allocation header, a resourceblock assignment, a TPC command for PUCCH, a downlink assignment index(applied to only a TDD operation), a HARQ process number and a HARQswapping flag. A modulation and coding scheme, a new data indicator andredundancy version information may be included with respect to codewords1 and 2. In addition, precoding information having a predeterminednumber of bits according to the number of antenna ports and atransmission mode of the base station may be included.

The number of bits of the precoding information in the 3GPP LTE systemis defined as follows.

TABLE 1 Transmission mode Number of antenna Closed-loop spatialOpen-loop spatial ports at base station multiplexing multiplexing 2 3 04 6 3

In the 3GPP LTE system, a maximum of two codewords can be simultaneouslytransmitted. A codeword field indicating whether a correspondingcodeword is activated or inactivated may be included in the DCI format2. However, the information indicating whether any codeword is activatedor not may be transferred implicitly. That is, the informationindicating whether any codeword is activated or not may be transferredby other control information. The analysis of the precoding informationhaving the number of bits of Table 1 may be changed according to thecodeword field indicating whether the codeword is activated orinactivated.

In the embodiment of the present invention, a method of efficientlyinforming the UE of the information about the PMI used by the basestation while the amount of additional control information is minimized,by adding a confirmation information to a portion of a combinationrepresented by the precoding information having a predetermined numberof bits which is differently analyzed depending on whether the codewordis activated or inactivated, is suggested. Since the confirmationinformation indicates that a PMI which is recently received from the UEis used by the base station, a temporal relationship between the PMIreported by the UE and the PMI used by the base station needs to beclearly defined, in order to prevent confusion between the base stationand the UE from occurring.

FIG. 3 is a view showing a temporal relationship between the PMIreported by a UE and confirmation information according to an embodimentof the present invention.

In FIG. 3, a horizontal axis denotes a time and . . . n-5, n-4, . . . ,n-1, n, . . . denote subframe indexes. As shown in FIG. 3, the UE mayfeed back the PMI to the base station for each subframe. If the basestation transmits the confirmation information to the UE on the basis ofan n^(th) subframe, the confirmation information indicates that the PMIreceived from UE is used by the base station. A predetermined processingtime may be consumed for enabling the base station to use the PMIreceived from the UE, as shown in FIG. 3. Thus, the PMI corresponding tothe confirmation information may be a PMI reported by the UE before apredetermined frame, for example, a PMI reported from the UE at ann-x^(th) subframe. At this time, “x” may be a predetermined integer oran integer determined according to upper layer signaling information.

Accordingly, in a preferred embodiment of the present invention, theconfirmation information indicates that the latest PMI received is usedby the base station. If a time point when the confirmation informationis transmitted is the n^(th) subframe, the confirmation informationindicates that the latest PMI reported via the n-x^(th) subframe isused. In more detail, in order to process the PMI received from the UE,a time of about 3 ms may be consumed. In this case, it is preferablethat x is predetermined as 4.

Here, “the confirmation information” may be referred differently, suchas “confirmation field” or just downlink control information entryinformation that the base station applied PMI received from UE tocorresponding precoding.

Meanwhile, the precoding information may have bit numbers which arechanged according to the number of antenna ports and transmission modeof the base station as shown in Table 1. At this time, the transmissionmode may include an open-loop spatial multiplexing mode and aclosed-loop spatial multiplexing mode as shown in Table 1.

In the open-loop spatial multiplexing mode, since a rank and a PMI areselectively transmitted without receiving feedback information from theUE, the advantage due to the addition of the confirmation informationcannot be obtained. Accordingly, in the present embodiment, theconfirmation information may be added only to the precoding informationof the closed-loop spatial multiplexing transmission mode. Accordingly,the detailed contents of the precoding information having the bitnumbers shown in Table 1 may be set as follows.

TABLE 2 One codeword: Two codewords: Codeword 1 enabled, Codeword 1enabled, Codeword 2 disabled Codeword 2 enabled Bit field Bit fieldmapped to mapped to index message index message 0 RI = 1: transmissiondiversity 0 RI = 2: PMI corresponding to precoding matrix$\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}$ 1 RI = 1: PMI corresponding to 1 RI = 2: PMI precodingvector [1 1]^(T)/{square root over (2)} corresponding to precodingmatrix $\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}$ 2 RI = 1: PMI corresponding to 2 RI = 2: confirmprecoding vector[1  −1]^(T)/{square root over (2)} the latest PMIreported via PUSCH 3 RI = 1: PMI corresponding to 3 Reserved precodingvector [1  j]^(T)/{square root over (2)} 4 RI = 1: PMI corresponding to4 Reserved precoding vector [1  −j]^(T)/{square root over (2)} 5 RI = 1:confirm the latest PMI 5 Reserved reported via PUSCH, If RI = 2 isreported, first column of all precoder represented by reported PMI andreported RI is used 6 RI = 1: confirm the latest PMI 6 Reserved reportedvia PUSCH, If RI = 2 is reported, second column of all precoderrepresented by reported PMI and reported RI is used 7 Reserved 7reserved

TABLE 3 One codeword: Two codewords: Codeword 1 enabled, Codeword 1enabled, Codeword 2 disabled Codeword 2 enabled Bit field Bit fieldmapped mapped to index Message to index message  0 RI = 1:  0 R = 2: PMI= 0 transmission diversity  1 RI = 2: PMI = 0  1 RI = 2: PMI = 1  2 RI =1: PMI = 1 . . . . . . . . 15 RI = 2: PMI = 15 . . . . 16 RI = 1: PMI =15 16 RI = 2: confirm the latest PMI reported via PUSCH 17 RI = 1:confirm the 17 RI = 3: PMI = 0 latest PMI reported via PUSCH 18 RI = 2:PMI = 0 18 RI = 3: PMI = 0 19 RI = 2: PMI = 1 19 RI = 3: PMI = 1 . . . .. . . . . . . . 33 RI = 2: PMI = 15 32 RI = 3: PMI = 15 34 RI = 2:confirm the 33 RI = 3: confirm the latest PMI latest PMI reported viaPUSCH reported via PUSCH 35~63 Reserved 34 R = 4: PMI = 0 35 RI = 4: PMI= 1 . . . . . . 49 RI = 4: PMI = 15 50 RI = 4: confirm the latest PMIreported via PUSCH 51~63 Reserved

Table 2 shows the contents of the precoding information of the2-antenna-port closed-loop spatial multiplexing transmission modeaccording to the present embodiment and Table 3 shows the contents ofthe precoding information of the 4-antenna-port closed-loop spatialmultiplexing transmission mode according to the present embodiment. InTables 2 and 3, the confirmation information is included in portionsdescribed by bold characters and the base station informs the UE thatthe PMI reported via the n-x^(th) subframe and more preferably ann-4^(th) subframe is used on the basis of the n^(th) subframe.

Hereinafter, the above-described embodiment will be described in detailaccording to the modes for transmitting the feedback information by theUE.

The information which is fed back to the base station by the UE includesChannel Quality Indicator (CQI) and PMI. In the 3GPP LTE system, the UEmay report the feedback information via a physical uplink shared channel(PUSCH) or a physical uplink control channel (PUCCH). Tables 4 and 5show the report types of the CQI/PMI for the PUSCH report mode and thePUCCH report mode.

TABLE 4 PMI Feedback Type No PMI Single PMI Multiple PMI PUSCH CQIWideband Mode 1-2 Feedback Type (wideband CQI) UE Selected Mode 2-0 Mode2-2 (subband CQI) Higher Layer- Mode 3-0 Mode 3-1 configured (subbandCQI)

TABLE 5 PMI Feedback Type No PMI Single PMI PUCCH CQI Wideband Mode 1-0Mode 1-1 Feedback Type (wideband CQI) UE Selected Mode 2-0 Mode 2-1(subband CQI)

In the PUSCH report mode shown in Table 4, the CRC may be attached inorder to ensure the reliable CQI/PMI report. Accordingly, as describedabove, in a downlink control channel, the confirmation informationindicating that the base station uses the PMI which is recently receivedfrom the UE may be used.

In the PUCCH report mode shown in Table 5, the CRC is not attached. Thebase station cannot check whether the CQI/PMI reported via the PUCCH isreliable or erroneous. In this case, in order to prevent confusion aboutthe PMI between the base station and the UE from occurring, explicit PMIindicating information is required in downlink.

Accordingly, the confirmation information suggested in theabove-described embodiment of the present invention considers only thePMI received via the PUSCH from the UE and does not consider the PMIreceived via the PUCCH.

Meanwhile, if the PUSCH data transmission and the PUCCH CQI/PMItransmission are simultaneously requested, PUCCH report bits may bepiggy-backed to the PUSCH transmission. A resource allocationrelationship of the PUCCH CQI/PMI report piggy-backed to the PUSCH willnow be described.

FIG. 4 is a view showing a resource allocation relationship in the casewhere a PUSCH in which data, a control channel, ACK/NACK and rankinformation are multiplexed is transmitted.

First, the data and the control channel are multiplexed in series. Inmore detail, first, the control channel is multiplexed and then the datais continuously multiplexed. The multiplexed control channel and data isfirst mapped to one symbol region by increasing a virtual subcarrierindex on a time axis and is then mapped to a next symbol region by thesame method. Rank information is mapped to a symbol separated from asymbol for transmitting a reference signal by one symbol, and ACK/NACKinformation is mapped to symbols adjacent to the symbol for transmittingthe reference signal from a symbol having a large virtual subcarrierindex to a symbol having a small virtual subcarrier index. FIG. 4 showsthe form in which the data, the control channel, the ACK/NACK and therank information are multiplexed according to the above-describedmapping order.

As shown in FIG. 4, if the PUSCH in which the data, the control channel,the ACK/NACK and the rank information are multiplexed is transmitted,the PUCCH CQI/PMI information piggy-backed to the PUSCH may betransmitted via a control channel region of FIG. 4.

If the PUSCH to which the PUCCH feedback information is piggy-backed istransmitted, the following matters are suggested in the presentembodiment.

First, in the present embodiment, the PUCCH feedback informationpiggy-backed to the PUSCH does not include the CRC. Since the explicitPMI indicating information transmission is included in the downlinkcontrol signaling, the PUCCH feedback information does not include theCRC. Since the PUCCH feedback information has as a small size of 10 to12 bits as a maximum, the attachment of the CRC having a size of 8 bitsmay excessively increase the overhead in order to search for an error.

Second, in the present embodiment, the PUCCH feedback informationpiggy-backed to the PUSCH is not considered as the PMI indicated by theconfirmation information. If the confirmation information is set in thedownlink control signaling of the n^(th) subframe, the UE assumes thatthe latest PMI reported by the UE is used by the base station before then-x^(th) subframe as described above. Accordingly, if the PUCCH feedbackinformation piggy-backed to the PUSCH is considered as the latest PMI asdescribed above, the PUSCH report mode has information having a largersize with respect to a channel status, system performance may seriouslydeteriorate.

Accordingly, in a preferred embodiment of the present invention, if theconfirmation information set in the precoding information of theclosed-loop spatial multiplexing transmission mode is transmitted via aspecific resource block (RB) of the n^(th) subframe, the confirmationinformation indicates that the PMI transmitted via the PUSCH of then-4^(th) subframe or a subframe prior thereto, that is, the PMI exceptfor the PMI transmitted via the PUCCH, is used by the base station.

Meanwhile, among the transmission modes of Table 4, Mode 1-2, Mode 2-2and Mode 3-1 corresponding to a single PMI transmission mode and a multiPMI transmission mode will be described in detail as follows.

First, Mode 1-2 corresponding to the wideband feedback will bedescribed. In Mode 1-2, preferred precoding matrixes are selected fromthe codebook on the assumption that transmission is made by onlycorresponding subbands. The UE reports one wideband CQI value percodeword, which is calculated when it is assumed that a selectedprecoding matrix is used in each subband and is transmitted by Ssubbands. The UE reports the selected PMI to each of the S subbands.

In addition, Mode 3-1 corresponding to the subband feedback configuredby an upper layer will be described. In Mode 3-1, one precoding matrixis selected from the codebook on the assumption that transmission ismade by S subbands. The UE reports one wideband CQI value per codewordfor the S subbands, which is calculated when it is assumed that thetransmission is made by the S subbands and one precoding matrix is usedin all the subbands. The UE reports the selected PMI to each of the Ssubbands. The UE transmits one PMI.

Finally, Mode 2-2 corresponding to the subband feedback selected by theUE will be described. In Mode 2-2, the UE selects M preferred subbandshaving a size k in a set of S subbands and selects a preferred precodingmatrix, which will be used for the transmission via the selected Msubbands, from the codebook. Thereafter, the UE may report one CQI percodeword in consideration of the case where transmission is made using aselected precoding matrix via the preferred M subbands as describedabove. The UE may report a preferred precoding matrix with respect tothe selected M subbands. On the assumption that the transmission is madevia the S subbands, one precoding matrix is selected from the codebook.The UE may report the wideband CQI per codeword on the assumption thatthe transmission is made by the S subbands and one precoding matrix isused in all the subbands. The UE may report a selected PMI with respectto all the S subbands.

The whole overhead when the transmission modes 1-2, 2-2 and 3-1 areimplemented by different system configurations is as follows.

TABLE 6 No. 5 MHz (25 RBs) 10 MHz (50 RBs) 20 MHz (100 RBS) Mode CW 2-TX4-TX 2-TX 4-TX 2-TX 4-TX 1-2 1 25 32 31 40 43 56 2 21 35 25 43 33 59 2-21 21 23 25 27 30 32 2 24 28 28 32 33 37 3-1 1 21 22 25 26 33 34 2 37 3945 47 61 63

In Table 6, the system band includes 5 MHz (25 RB), 10 MHz (50 RB) 20MHz (100 RB) and the overhead according to each transmission mode isrepresented by the bit number in 2-transmission-antenna (2 Tx) and4-transmission-antenna (4 Tx).

In Table 6, only the information bit is considered in each transmissionmode. Accordingly, if an additional redundant bit for checking an error,such as the CRC, is included, the information bit needs to beconsidered.

Generally, in downlink, the base station represents only theconfirmation information of one PMI via the confirmation field. In thiscase, in Mode 3-1 corresponding to the single PMI transmission mode,since the base station can use one PMI fed back from the UE regardlessof whether or not the error exists, the advantage due to the use of theconfirmation information is reduced in order to reduce the overhead. Incontrast, in Modes 1-2 and 2-2 corresponding to the multi PMItransmission mode, the advantage due to the transmission of theconfirmation information indicating that the PMIs transmitted from theUE are used by the base station can be obtained. Information indicatingwhich of the PMIs is used can be obtained by clarifying the temporalrelationship between the feedback of the PMI and the transmission of theconfirmation information as described with reference to FIG. 3. Thus, itis possible to prevent confusion between the base station and the UEfrom occurring.

According to the embodiment, if the CRC is used only in Modes 1-2 and2-2 corresponding to the multi PMI transmission mode, the whole overheadis as follows.

TABLE 7 No. 5 MHz (25 RBs) 10 MHz (50 RBs) 20 MHz (100 RBS) Mode CW 2-TX4-TX 2-TX 4-TX 2-TX 4-TX 1-2 1 25 + L 32 + L 31 + L 40 + L 43 + L 56 + L2 21 + L 35 + L 25 + L 43 + L 33 + L 59 + L 2-2 1 21 + L 23 + L 25 + L27 + L 30 + L 32 + L 2 24 + L 28 + L 28 + L 32 + L 33 + L 37 + L 3-1 121 22 25 26 33 34 2 37 39 45 47 61 63

In Table 7, “L” denotes the bit number of the CRC, which may be changedaccording to the bandwidth and/or the size of the information bits. Forexample, as the CRC attached to the modes 1-2 and 2-2, a CRC having alength of 16 bits may be used. A polynomial of generating such a CRC maybe expressed as follows.

g _(CRC16)(D)=[D ¹⁶ +D ¹² +D ⁵+1]foraCRClengthL=16  Equation 1

FIG. 5 is a diagram explaining an example of a method of transmittingdownlink control information by the base station in accordance with oneembodiment of the present invention. Referring to FIG. 5, the basestation receives feedback information including a PMI from the UE.(S501) The base station applies a precoding to a downlink signal andtransmits the downlink signal to the UE. (S502) And then, the basestation transmits precoding information having a predetermined number ofbits according to a number of antenna ports and a transmission mode ofthe base station to the UE. (S503)

Here, the precoding to the downlink signal for a specific resource block(RB) in n^(th) subframe is according to a latest PMI received from theUE on or before n-x^(th) subframe. “x” is a predetermined integer or aninteger determined by upper layer signaling higher than a physicallayer.

A CRC having a length of 8, that is, L=8, may be attached to the modes1-2 and 2-2.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

As described above, according to a method for transmitting and receivingdownlink control information of the embodiments of the presentinvention, it is possible to efficiently inform a user equipment ofinformation about PMI used by an base station using a small amount ofprecoding information and minimize the overhead while maintaining systemperformance by efficiently setting the CRC attachment.

Such a method is applicable to various systems such as a 3GPP LTE-Asystem and an IEEE 802.16 series system as well as the 3GPP LTE systemby the same principle.

1. A method for transmitting downlink signals to an user equipment (UE)by a base station, the method comprising: receiving at least oneprecoding matrix index (PMI) from the UE; and transmitting a downlinkcontrol signal and a downlink data signal applied a precoding, wherein,if the at least one PMI is received via a physical uplink shared channel(PUSCH) and is not piggy-backed to the PUSCH, the downlink controlsignal includes a first message indicating that the base station hasapplied the precoding using the at least one PMI received from the UE.2. The method according to claim 1, wherein, if the at least one PMI isreceived via a physical uplink control channel (PUCCH) or ispiggy-backed to the PUSCH, the downlink control signal includes a secondmessage indicating which PMI is used for the precoding by the basestation.
 3. The method according to claim 2, wherein a cyclic redundancycheck (CRC) is attached to the at least one PMI received via the PUSCH,and the CRC is not attached to the at least one PMI received via thePUCCH.
 4. The method according to claim 4, wherein the CRC is notattached to the at least one PMI which is piggy-backed to the PUSCH. 5.The method according to claim 1, wherein the downlink data signal istransmitted using a closed-loop spatial multiplexing transmission mode.6. The method according to claim 1, wherein the downlink control signaland the downlink data signal are transmitted in n^(th) subframe, and theat least one PMI is received on or before n-4^(th) subframe.
 7. A methodfor receiving downlink signals from a base station by a user equipment(UE), the method comprising: transmitting at least one precoding matrixindex (PMI) to the base station; and receiving a downlink control signaland a downlink data signal from the base station, wherein the basestation has applied a precoding to the downlink data signal, wherein, ifthe at least one PMI is transmitted via a physical uplink shared channel(PUSCH) and is not piggy-backed to the PUSCH, the downlink controlsignal includes a first message indicating that the base station hasapplied the precoding using the at least one PMI.
 8. The methodaccording to claim 7, wherein, if the at least one PMI is transmittedvia a physical uplink control channel (PUCCH) or is piggy-backed to thePUSCH, the downlink control signal includes a second message indicatingwhich PMI is used for the precoding by the base station.
 9. The methodaccording to claim 7, wherein transmitting the at least one PMIcomprises attaching a cyclic redundancy check (CRC) to the at least onePMI transmitted via the PUSCH, and wherein the CRC is not attached tothe at least one PMI transmitted via the PUCCH.
 10. The method accordingto claim 9, wherein the CRC is not attached to the at least one PMI inthe PUCCH which is piggy-backed to the PUSCH.
 11. The method accordingto claim 7, wherein the downlink data signal is transmitted using aclosed-loop spatial multiplexing transmission mode by the base station.12. The method according to claim 7, wherein the downlink control signaland the downlink data signal are received in n^(th) subframe, and the atleast one PMI is transmitted on or before n-4^(th) subframe.